In most fuel injectors, fuel is pressurized within the injector body by a reciprocating plunger. In one class of fuel injection systems, this plunger is driven downward during its pumping stroke by utilizing a relatively high pressure hydraulic actuation fluid. While virtually any available fluid, including fuel fluid, could conceivably be used as the hydraulic medium in such a fuel injector, Caterpillar, Inc. of Peoria, Illinois has encountered considerable success in utilizing engine lubricating oil as the hydraulic medium in hydraulically-actuated fuel injectors. An example of such a hydraulically-actuated electronically-controlled (HEUI) fuel injector is shown for example in U.S. Pat. No. 5,213,083 to Glassey.
In a typical HEUI fuel injector, the flow of high pressure actuation fluid to an individual injector is controlled by a solenoid actuated control valve attached to each individual injector. Each injection event is initiated when an engine electronic control module commands energization of the solenoid actuated control valve to move it to open an injector's hitch pressure actuation fluid inlet. As high pressure actuation fluid (oil) flows into the injector, the internal plunger is driven downward to pressurize fuel. When the fuel reaches a valve opening pressure, a needle valve member opens and fuel commences to spray out of the nozzle outlet of the individual injector. Each fuel injection event is ended by de-energizing the solenoid actuated control valve to close the high pressure actuation fluid inlet. This in turn ends the downward pumping stroke of the internal plunger and causes fuel pressure to drop. When the fuel pressure drops below a certain pressure, the needle valve member closes the nozzle outlet, and the injection event is ended. Between injection events, some biasing means, such as a return spring, retracts the plunger for a subsequent injection evtent.
While these basic HEUI fuel injectors have performed magnificently for many years, engineers are continuously looking for ways to improve the same. It has become well known that particulate and NOx emissions from a diesel engine can be significantly reduced if one has the ability to control an injection rate profile independent of engine operating conditions. For instance, one injection profile will optimize emissions at idle conditions, whereas a completely different injection rate profile will optimize emissions at a high rpm fully loaded condition for a particular diesel engine. Injection rate profiLes have generally been divided into four different groups including pilot injection, boot shaped injection, ramp-square injection and square injection. In almost all cases, engineers have discovered that emissions can be significantly improved if each injection event can be ended as abruptly as possible.
In order to improve control over injection rate shaping and hence the ability to reduce undesirable emissions in a diesel engine utilizing a HEUI type fuel injection system, Caterpillar, Inc. introduced the concept of a directly operated needle valve in HEUI type fuel injectors. This concept is disclosed, for example, in U.S. Pat. No. 5,463,996 to Maley et al. In this type of fuel injector, a first solenoid control valve controls the opening and closing of the high pressure actuation fluid inlet and a second solenoid control valve controls the opening and closing of the needle valve member. In this fuel injector, the needle valve member is modified to include a closing hydraulic surface on one end that is exposed to fluid pressure in a needle control chamber. The needle control valve opens the needle control chamber to one of either a source of high pressure fluid or a Low pressure passage. The injector is designed such that the needle valve member will not open or will abruptly close whenever the needle control chamber is opened to the source of high pressure fluid. When the needle control chamber is open to the low pressure passage, the needle valve member behaves as a conventional needle check valve. While the innovation of direct control over the needle valve permits a significant improvement in injection rate shaping, the inclusion of two separate solenoid actuated control valves is less than desirable from both a cost and reliability or robustness standpoint.
The present invention is directed to improving upon the hydraulically-actuated electronically-controlled fuel injection systems of the prior art.